1. Field of the Invention
The present invention relates to thermostatic control systems for regulating the ambient temperature of an enclosed area such as the living space of a home or office, and in particular to a digital electronic thermostat for automatically controlling the operation of heat transfer apparatus in which the thermostat cycles the heat transfer apparatus on and off to maintain the ambient temperature within a living space at either an energy conserving temperature level or a comfort temperature level during selected periods of night and day, respectively, and at the energy conserving temperature level during selected set-back periods.
2. Description of the Prior Art
It is well known in air conditioning and heating systems to utilize a thermostat which is manually settable to a selected control temperature level for regulating the ambient temperature of a living space in a home, office or other building. Conventional thermostatic controls include electro-mechanical devices which automatically cycle the furnace, air conditioning unit and related equipment between "on" and "off" states to maintain the temperature at the selected level. Such conventional electro-mechanical thermostatic control units typically comprise a nonpowered component such as a temperature sensitive bimetal switch or a bimetal thermometer. These electro-mechanical thermostatic controls are responsive to ambient temperature and are usually installed in the low voltage control circuit of a furnace or air conditioner. The regulated temperature is determined by manually setting a set point device on the thermostatic control by which the temperature is regulated within a few degrees over a range of from about 50.degree. F. to 90.degree. F. The thermostatic control is typically mounted upon a centrally located wall within the dwelling or office and is coupled to the furnace or air conditioning unit by means of permanent wiring located within the wall.
For ideal comfort or for energy conservation reasons, it is common practice to reduce the degree of cooling or heating during periods of minimum activity, for example during nighttime or at other times of minimum activity, according to comfort requirements. For example, the ambient temperature in a home can be reduced significantly at night when the occupants are asleep without causing any discomfort. Additionally, the temperature can be reduced substantially during the day when the dwelling is not occupied. Such a reduction in the nighttime temperature of the dwelling is referred to as a "set-back" which results in a significant reduction in fuel consumption and heating or cooling costs for the dwelling. With the conventional electro-mechanical thermostat, such temperature set-backs have been carried out manually. The need to manually alter the regulated temperature is subject to the human failure of forgetting to change the thermostatic setting whereby cost savings are lost. Also, manual alteration of the regulated temperature is not entirely satisfactory since the dwelling will be uncomfortably cool in the morning before it is manually reset and, due to the lag time of typical heating systems, time is required for the temperture of the dwelling to rise to a comfortable level for normal daytime activities.
Automatic set-back has been provided by a conventional electromechanical thermostat arrangement for regulating room temperature at two different temperature levels in a predetermined time sequence. Such arrangements typically provide a timer for night set-back and require two separate sources of energy, one for the timer and another for the thermostat circuitry. An additional transformer or an external battery is usually provided for energizing the timer. A pair of temperature sensitive switches is also required for manually setting the two control levels. One switch is manually set to a desired day temperature and the other is manually set to a desired night temperature. The timer automatically enables one of the switches in the morning, and enables the other in the evening. The cost and need for installing an additional power circuit for conventional set-back thermostats represents a practical limitation on the use of such thermostats as a replacement for the single cycle electro-mechanical thermostat control in existing structures.
An additional limitation on the adaptation of electro-mechanical thermostat/timer combinations for regulating ambient temperature at two different temperature levels and for providing either day set-back or night set-back is the requirement of a large number of data input control circuits for manually setting time and temperature set point information. For example, a thermostatic control unit having night set-back, day set-back and day skip capability requires the following data inputs:
(a) selection of daytime temperature over a 40.degree.-50.degree. F. temperature range;
(b) selection of nighttime temperature over a 40.degree.-50.degree. F. temperature range;
(c) selection of day-to-night transition time;
(d) selection of night-to-day transition time;
(e) selection of time for initiating day set-back;
(f) selection of time for terminating day set-back;
(g) selection of day or days of the week to skip; and,
(h) other special instructions such as mode selection (Cool--Off--Heat) and fan operation (On--Auto).
The number of conductors and switches required to implement the foregoing functions throughout the desired time and temperature ranges in an electro-mechanical thermostat control unit, either with or without timers, would necessarily require a large number of conductors and switches within a relatively large housing. Such information could be fed into a thermostatic controller in digital form with a keyboard input device, but to gain broad market acceptance, minimize setting errors and have all the settings apparent or intuitively obvious at all times, a "sliding bar" type of input is desirable. With such an arrangement, less operator training is required, less reliance on instruction manuals is needed, and in business operations, the settings can be determined through a clear plastic cover which may be locked over the thermostatic control unit to prevent tampering.
However, with a conventional sliding bar type of data input arrangement, to bring out the large number of conductors necessary for encoding the position of the sliding bar switches would mean that the circuit receiving the inputs would necessarily require an inordinately large capacity to be compatible with the required number of data input nodes. Therefore for domestic applications where a minimum size and appearance are important considerations, it has not been practical to implement the coventional sliding bar type arrangement, even though it provides the aesthetically pleasing, intuitively obvious indication of input setting.